1. Technical Field
This disclosure relates to a reversible thermosensitive recording material, an IC card, a magnetic card and a method for producing a reversible thermosensitive recording material.
2. Description of the Related Art
Thermosensitive recording materials utilizing color-developing reaction between electron-donating color-forming compounds (hereinafter otherwise referred to as “color formers” or “leuco dyes”) and electron-accepting compounds (hereinafter otherwise referred to as “developers”) are widely known, and they are widely used as output paper for facsimiles, word processors, scientific measurement devices, etc. along with the development of office automation and also as magnetic cards and IC cards such as commutation tickets for transportation, prepaid cards and discount cards. In particular, these days, note is taken of development of reversible thermosensitive recording materials capable of being rewritten as many times as desired, in view of ecological problems, waste problems and the like.
Reversible color development and color erasure of a reversible thermosensitive recording material will be briefly explained. A typical reversible thermosensitive recording material includes a support, and a thermosensitive recording layer formed on the surface of the support, wherein the support is made of paper, a plastic card, etc. in the form of a film, sheet or plate, and the thermosensitive recording layer is formed of a composition in which the color former and the developer are mixed and dispersed in a thermoplastic resin or the like. The composition containing the color former and the developer in this thermosensitive recording layer does not develop color when the color former and the developer are simply mixed together in a solid form. However, when this composition is made high in temperature, the whole of it comes into a melting state, and the color former and the developer contained react together and develop color. When the composition in this melting state is slowly cooled, the color former and the developer dissociate from each other in the vicinity of the melting temperature thereof, and each of them separately flocculates or crystallizes, thereby erasing the color. Then that state is brought into a frozen state by the solidification of the thermoplastic resin, etc. serving as a binder. However, if the composition in a melting state is rapidly cooled, the composition comes into a frozen state and solidifies before the color former and the developer dissociate from each other. Thus, a color-developed state is maintained, with the color former and the developer bonded together. Selection of a composition which has an appropriate melting temperature and an appropriate freezing temperature and which is formed from a combination of a binder and two types of compounds inducing such a phenomenon makes it possible to select color development and color erasure based upon adjustment of the cooling rate after the composition has been melted by heating, and to keep each of the color-developed state and the colorless state in a frozen state at normal temperature.
FIG. 6 employs a graph to show a change in color development and color erasure of the thermosensitive recording material with respect to a temperature change. In FIG. 6, the horizontal axis denotes time, while the vertical axis denotes temperature. “T1” denotes the melting reaction temperature of the color former and the developer, and “T2” denotes the temperature at which the composition composed of the color former, the developer and the binder solidifies into a frozen state. In other words, in the temperature range between the T1 and the T2, the composition can be flocculated or crystallized, with the color former and the developer dissociated from each other. However, it takes some time for the composition to be flocculated or crystallized through dissociation.
On the graph, the composition originally in a state (a) (color-developed state) at normal temperature is heated to the temperature T1. When the composition has a temperature of T1, it melts within a time t1 and maintains a color-developed state (b) while melting. The composition is slowly cooled, and the temperature thereof is reduced to a temperature T2 with a time t2 being spent, and then returned to normal temperature. When the time t2 is equal to or longer than the time spent by the composition, which has been developing color with reaction, in dissociating into the color former and the developer and flocculating or solidifying, the composition dissociates and comes into a colorless state (c).
When the composition which has lost the color is reheated into a melting state (d), the composition develops the color as the color former and the developer melt and react together. If this composition is rapidly cooled to normal temperature within a short time t4, the composition remains color-developed, as its reacting molecules remain in a frozen state (e).
Further, when the composition in the state (e) is exposed to the dissociation and crystallization temperature range between the melting temperatures T1 and T2 for a long time t5, the color former and the developer dissociate from each other and flocculate or crystallize, thereby possibly erasing the color. In this case as well, when the temperature of the composition is returned to normal temperature, the composition remains in a colorless state (g). By utilizing such a phase change of the composition, it is possible to make the composition develop color and lose color based upon heating and cooling temperatures and rate control. Note that although the space between the T1 and the T2 on the graph is schematically shown and seen large, compositions having approximately several degrees Celsius to 10° C. as this temperature range are applicable in reality.
In Japanese Patent (JP-B) No. 2,981,558, the present inventors proposed a reversible thermosensitive color-developing composition in which an organic phosphoric acid compound having a long-chain aliphatic hydrocarbon group, an aliphatic carboxylic acid compound, or a phenol compound is used as a developer, and a leuco dye serving as a color former is combined therewith; and a reversible thermosensitive recording material using the reversible thermosensitive color-developing composition as a recording layer. This reversible thermosensitive recording material makes it possible to carry out color development and color erasure easily based upon adjustment of heating conditions, sustain the color-developed state and the colorless state stably at normal temperature and repeat color development and color erasure.
In principle, a reversible thermosensitive recording material only requires a thermosensitive recording material layer capable of repeating color development and color erasure as described above. However, as to a reversible thermosensitive recording material disclosed in JP-B No. 2,981,558, a leuco dye used in a reversible thermosensitive recording layer has such a problem that a color-developed portion may fade or a non-color-developed portion (colorless portion) may change in color, thereby impairing the whiteness. The present inventors have found that fading or discoloration of a reversible thermosensitive recording material is related to a tiny amount of oxygen. In particular, many of leuco dyes used as color formers are liable to induce radical reaction with oxygen when activated by light. When a radical reaction is induced, a thermosensitive recording material layer, which has been developing color, may lose the color or may fade; also, a thermosensitive recording material layer, which has been in a colorless state, may develop color, for example, in such a manner as to become yellow.
In each of JP-B Nos. 3,501,430 and 3,504,035, in order to remove such fading of a color-developing portion and discoloration of a non-color-developed portion as described above, the present inventors proposed a reversible thermosensitive recording material in which a thermosensitive recording layer formed of a leuco dye that is relatively resistant to light exposure is covered with a gas barrier layer formed of a polymer resin that has an oxygen-insulating function. Further, in each of JP-B Nos. 3,549,131 and 3,596,706 and JP-A No. 06-1066, the present inventors proposed adding an antioxidant such as α-tocopherol or a vitamin into a gas barrier layer formed of a polymer resin. Such techniques yielded improvements in preventing fading of color-developing images and securing background whiteness. However, as the components are used as a reversible thermosensitive recording material for a long time and repeatedly subjected to heating and cooling for recording and erasure, there are such problems arising that damage to the gas barrier polymer film accumulates, which may cause the gas barrier layer to detach, and thus there may be a degradation of gas barrier function.
In each of JP-A Nos. 09-175024, 2006-82252 and 2006-88445, in an attempt to remove the problem of detachment of a gas barrier layer, there is a proposal to provide between a thermosensitive recording layer and a gas barrier layer an adhesive layer formed of a water-soluble resin or the like, and a proposal to improve the properties of an adherend by adding a specific adhesive into a gas barrier layer, and relatively favorable improvements are yielded.
As discussed supra, reversible thermosensitive recording materials generally have gas barrier layers for oxygen insulation. The gas barrier layers are films formed by synthetic polymer resins having typical gas barrier functions. Among such synthetic polymer resins, polyvinyl alcohol (PVA) resins are characterized in that they are flexible and antistatic and also they are superior in gas barrier property when dry. However, PVA resins have a high affinity for moisture, their gas barrier functions depend largely upon humidity when they are provided as gas barrier films, and thus their gas barrier properties degrade noticeably under high-humidity conditions.
In order to reduce the moisture absorption properties of PVA resins, hydroxyl groups of PVA can be subjected to a chemical modification such as acetalization for water resistance. However, although PVA may be able to be made water resistant by doing so, the hydrogen bonding strength of hydroxyl groups serving as a gas barrier exhibition mechanism of PVA decreases, and thus the original gas barrier properties of PVA are noticeably impaired. Meanwhile, ethylene-vinyl alcohol (EVOH) copolymers, which are materials with gas barrier functions, are superior to PVA in water resistance but inferior to PVA in hydrogen bonding strength, and thus their gas barrier properties cannot be adequately secured under high-humidity conditions.
There has not yet been realized a reversible thermosensitive recording material which does not cause fading of a recorded image or yellowing of a background under high-humidity conditions and in an environment where the material is exposed to fluorescent light or sunlight.